1. Field of the Invention
The present invention relates to a sheet conveying device, more particularly to a sheet conveying device that conveys a sheet in an image forming apparatus, such as a printer and an MFP (Multifunction Peripheral).
2. Description of the Related Art
Examples of an electrophotographic image forming apparatus include the MFP having a scanner function, a facsimile function, a copying function, a function as a printer, a data communication function, and a server function, a facsimile machine, a copying machine, and the printer.
For example, an image is formed on a sheet by the following method in a general image forming process in which an electrophotographic system is adopted. A toner image is formed on a surface of an image bearing member by an electrostatic recording method, and the toner image is transferred to an intermediate transfer belt by a primary transfer roller, and conveyed to a secondary transfer roller. The sheet is fed from a feed cassette one by one, and conveyed to the secondary transfer roller in at a predetermined time. The toner image on the intermediate transfer body is electrostatically transferred to the sheet conveyed to the secondary transfer roller. Then the sheet is conveyed to a fixing device. Heat and a pressure are applied to the sheet by the fixing device, thereby fixing the toner image. The sheet to which the toner image is fixed is discharged to a catch tray by a sheet conveying device.
The sheet conveying device includes a plurality of rollers (conveying rollers) disposed along a sheet passage route. The conventional sheet conveying device performs conveyance sequence control. That is, positional information on a position of the sheet conveyed along the sheet passage route is calculated from a sheet feed amount (the number of revolutions of a roller) by each roller, and the rollers are sequentially driven or stopped based on the positional information on the sheet. For example, the sheet conveying device starts the drive of the downstream-side roller when detecting that a leading end of the sheet arrives at the roller on the downstream side of the sheet passage route from the positional information on the sheet. The sheet conveying device stops the drive of the upstream-side roller when detecting that a tailing end of the sheet passes by the roller on the upstream side of the sheet passage route from the positional information on the sheet.
However, conventionally the positional information on the sheet is calculated based only on the sheet feed amount by the roller. Therefore, in the case that the sheet slips on the roller, only the positional information on the sheet is updated, and a deviation is generated between the actual position of the sheet and the positional information on the sheet. Although the leading end of the sheet does not actually arrive at the downstream-side roller, the downstream-side roller is driven while the upstream-side roller is stopped, and a conveyance mistake is generated between the upstream-side roller and the downstream-side roller, which results in a jam.
Conventionally, in order to prevent the generation of the jam due to the deviation between the actual position of the sheet and the positional information on the sheet, a sensor (conveyance sensor) that detects the arrival of the sheet is disposed at a point (a point at which a transfer of the drive is generated between the conveying rollers) at which drive states of the upstream-side roller and the downstream-side roller are switched in the sheet passage route. The sheet conveying device switches the drive states of each roller based on the detection state of the sensor in addition to the positional information on the sheet. Specifically, when the sensor provided near the downstream-side roller detects the leading end of the sheet, the sheet conveying device starts the drive of the downstream-side roller, and the downstream-side roller starts to convey the sheet.
A conveyance control program in a section as described above including the point at which the drive states of the upstream-side roller and the downstream-side roller are switched includes a sheet position management module that calculates, updates and retains the positional information on the sheet and a sequence module that refers to the positional information on the sheet to sequentially drive each rollers. The sequence module includes a sheet feeding sequence module at each sheet feeding port, which feeds the sheet from a feed cassette to convey the sheet to a downstream-side vertically conveying roller of the sheet feeding roller, and a vertically conveying sequence module that conveys the sheet from an upstream-side vertically conveying roller to a downstream-side vertically conveying roller as a sub-module according to a sheet conveying section.
For example, Documents 1 and 2 disclose technologies related to the sheet conveyance. Document 1 discloses the technology, in which a plurality of switches are included to detect the sheet and a paper location is updated based on the sheet detection states of the switches. In the technology disclosed in Document 2, a progression permitting unit calculates the time the next sheet is fed from a sheet size and a conveying path length and informs a transfer sheet unit of the sheet feeding time, and the transfer sheet unit receives a message from the progression permitting unit to control movement and stop of a print sheet.    Document 1: Japanese Patent Publication Laying-Open No. 62-93156    Document 2: Japanese Patent Publication Laying-Open No. 11-343064
In the case that a developer who develops each module of the sheet conveying device develops a plurality of models, it is necessary to design a position in which the drive of the roller is started in each model in order to prevent the conveyance mistake. It is necessary that each module refer to not only the positional information on the sheet but also the detection state of the sensor at the point at which the drive states of the upstream-side roller and the downstream-side roller are switched. As a result, unfortunately a device configuration becomes complicated. The problem cannot be solved in Documents 1 and 2.
The detection state of the sensor depends on the position in which the sensor is disposed and the number of sensors. Because the plurality of points at each of which the drive states of the upstream-side roller and the downstream-side roller are switched exist in one model, it is necessary for the developer to change a setting based on the detection state of the sensor at each point, and the developer takes a lot of trouble with production of the module. Because the position in which the sensor is disposed and the number of sensors vary in each model, the developer further takes a lot of trouble with the production of the module in the case that the plural models are developed.
As described above, the conventional sheet conveying device is designed depending on the detection state of the sensor that is changed in each model of the image forming apparatus. Therefore, the sheet conveying device is designed to be specialized only in a specific model, the design depending on the sensor state obstructs the development of the sheet conveying device (the sequence module) common to plural models to reduce development efficiency.